An engineering model for the thixotropic rheology of alumina (Al(2)O(3)) concentrated colloidal gels is proposed and compared to rheological experimental results. Concentrated colloidal gels used in extrusion-based solid freeform fabrication are often modeled as simple shear-thinning fluids with finite yield stress such that their rheological properties depend only on instantaneous shear rate and not on shear history. Although this is a satisfactory view for steady state flow, it lacks the detail to consider phenomena associated with transient processes such as die entry and exit in extrusion processes or the shape evolution of an extruded filament as it comes to rest in a low shear environment. The rheological model proposed here considers that both elastic and viscous properties of an aqueous Al(2)O(3) gel vary with microstructure changes of colloidal aggregates and that the aggregate structure is a function of both shear rate and time. A first-order structural kinetics assumption is used to quantify the microstructure evolution of the gel network during shear flow, where shear-induced attrition and diffusion limited aggregation are the rate limiting mechanisms. A constitutive flow equation is developed by incorporating the structural kinetics to describe unsteady shear-thinning behavior. The data collected through shear rate step-change measurements are used to determine 12 model parameters in the proposed rheological model. The model is compared and validated through hysteresis-loop experiments with satisfactory agreement. This model is quasiempirical in the sense that details of interparticle pair-potentials are not used in the formulation. Nevertheless, the model could have practical uses in simulation of the macroscopic flow behavior of colloidal gels during extrusion processes. Specifically, the model offers predictions of elastic and viscous property evolution of colloidal gels as a function of shear history. (C) 2011 The Society of Rheology. [DOI: 10.1122/1.3573828]